The above type of front derailleur for a bicycle has hitherto been well-know, and is disclosed in, for example, Japanese Utility Model Publication Gazette No. Sho 55-8,707. This front derailleur is provided with a fixing member fixed to the bicycle frame, a chain guide having an outer guide plate and an inner guide plate, and linkage members comprising a pair of parallel link plates. The linkage members are connected to the fixing member and chain guide through pivot pins extending perpendicularly to the axis of the multistage front gears to thereby form a linkage mechanism for allowing the chain guide to move axially of the front gear. A control wire is pulled to deform the linkage mechanism so as to move the chain guide axially of the front chain gear, thereby shifting the chain to a selectable one of the multistage front gears for changing the bicycle speed.
The front derailleur for the bicycle is provided with a return spring between the linkage member and the fixing member, by which the chain guide is biased toward the bicycle frame, that is, from a larger diameter front gear to a smaller diameter one. The control wire is pulled against the return spring to forcibly move the chain guide, thereby shifting the chain from the smaller diameter front gear to the larger diameter one, and the control wire is loosened to return the chain guide by the restoring force of the return spring, thereby shifting the chain to the smaller diameter gear.
In the front derailleur for the bicycle as described above, mention when the chain is shifted from the smaller diameter front gear to the larger diameter gear to increase the bicycle speed, the control wire is pulled against the return spring to forcibly move the chain guide. This is not so problematical. However, when the chain is shifted from the larger diameter front gear to the smaller diameter gear to reduce the bicycle speed, particularly, the chain guide is adapted to move only by a restoring force of the return spring, the force being applied to the chain to the smaller diameter front gear becomes small if the restoring force of the return spring is small. Furthermore a chain urging portion of the chain guide, when the bicycle speed is reduced, moves away from the engaging point of the chain with the front gear, thereby often causing a poor speed change efficiency.